The present invention relates to a light-emitting semiconductor device, such as a light-emitting diode, having a pn junction.
One conventional type of light-emitting diode (LED) is formed on the semiconductor substrate 30 shown in FIG. 34, comprising an n-type gallium-arsenide (GaAs) substrate layer 31 and an n-type gallium-arsenide-phosphide (GaAsP) epitaxial layer 32. Zinc (Zn) is selectively diffused into the epitaxial layer 32 to create a p-type diffusion area 33. The device is covered with an inter-layer insulating film 34, leaving a window through which a p-electrode 35 makes contact with the p-type diffusion area 33. An n-electrode 36 is formed on the underside of the device, in contact with the GaAs substrate layer 31. When a forward voltage is applied between the p-electrode 35 and n-electrode 36, light is emitted by recombination of carriers in the vicinity of the pn junction between the p-type diffusion area 33 and n-type epitaxial layer 32. In some LEDs, an additional semiconductor contact layer is formed as the uppermost layer of the substrate 30, to ensure ohmic contact between the p-electrode 35 and p-type diffusion area 33.
A problem in this conventional LED is that the p-electrode 35 blocks part of the emitted light. When the dimensions of the p-type diffusion area 33 are very small, the p-type electrode 35 may block most of the light. The problem is aggravated if the sheet resistance of the p-type diffusion area 33 is low, because then most of the driving current flows straight downward, and most of the light is emitted directly below the p-type electrode 35. Attempts to solve this problem by reducing the relative size of the p-electrode 35 have failed, because when the area of contact between the p-electrode 35 and p-type diffusion area 33 is reduced, the contact resistance is increased, and if the contact area is too small, the p-type electrode cannot supply enough current to drive the device at the small driving voltages typical of integrated driving circuits.
This problem occurs in LED arrays used as light sources in high-resolution printers, such as printers printing one thousand two hundred dots per inch (1200 dpi), for example. A further problem in such arrays is that variations in the contact area from one LED to another in the array create differences in driving current, leading to non-uniform light emission and undesirable printing irregularities.
If the substrate 30 has an uppermost semiconductor contact layer to assure ohmic contact, another problem occurs: the semiconductor contact layer absorbs light, reducing the emission efficiency of the device.
It is accordingly an object of the present invention to increase the light-emitting efficiency of a light-emitting semiconductor device.
According to a first aspect of the invention, a light-emitting semiconductor device comprises a semiconductor substrate having a plurality of semiconductor layers of a first conductive type, these layers including at least a light-emitting layer with a first bandgap energy, and an upper cladding layer, disposed above the light-emitting layer, with a second bandgap energy exceeding the first bandgap energy. A first diffusion area is formed by diffusion of an impurity of a second conductive type from the upper surface of the semiconductor substrate into the upper cladding layer and the light-emitting layer. A second diffusion area, continuous with the first diffusion area, is formed by diffusion of the same impurity from the upper surface into the upper cladding layer, but not into the light-emitting layer. A first electrode makes contact with a portion of the substrate outside the first and second diffusion areas. A second electrode, disposed outside the first diffusion area, makes contact with the upper surface of the substrate in the second diffusion area.
In the first aspect of the invention, light is emitted from the surface of the first diffusion area. The light is not blocked by the second electrode, and both the light-emitting area and the area of contact between the second diffusion area and second electrode can be adequately large.
According to a second aspect of the invention, a light-emitting semiconductor device comprises a semiconductor substrate having an upper surface, a semiconductor contact layer disposed at the upper surface, and a plurality of semiconductor layers of a first conductive type disposed below the upper surface. The layers of the first conductive type include at least a light-emitting layer with a first bandgap energy, and an upper cladding layer, disposed above the light-emitting layer, with a second bandgap energy exceeding the first bandgap energy. A diffusion area is formed by diffusion of an impurity of a second conductive type from the upper surface into the semiconductor contact layer, the upper cladding layer, and the light-emitting layer. One part of the diffusion area is a contact area. The portion of the semiconductor contact layer disposed in the diffusion area exterior to the contact area is removed by etching. A first electrode makes contact with the semiconductor substrate in an area outside the diffusion area. A second electrode makes ohmic contact with the contact area.
In the second aspect of the invention, light is emitted from an area including the part of the diffusion area not covered by the second electrode. Light emission is enhanced by removal of the semiconductor contact layer in this area.
The two aspects can be combined by providing a semiconductor contact layer in the first aspect of the invention, the semiconductor contact layer being completely removed by etching from the first diffusion area.